1. Technical Field
The present invention relates generally to processes for catalytic conversion of formamides and oxygen to isocyanates.
2. Description of the Prior Art
Heyboer, U.S. Pat. No. 4,207,251, discloses a method for gas phase production of C.sub.1-24 organo-isocyanates involving oxidative dehydrogenation of corresponding N-monosubstituted formamides. The product stream obtained contains isocyanate, water, nitrogen or other inert gas, unreacted formamide, and residual by-products. This reference discloses several methods for separation of product isocyanates from water, which are summarized below.
A first method involves rapid cooling. Upon termination of the oxidative dehydrogenation reaction, the reaction mixture can be rapidly cooled, whereupon the water-containing phase and isocyanate-containing phase are separated by filtration or extraction. As noted by the Heyboer patent, disadvantages of this method include the high cost of rapid cooling, and the risk of reaction of water and isocyanate where cooling is insufficient and separation is not accomplished sufficiently rapidly.
A second method requires contacting the reaction mixture with a water-absorbing agent prior to condensation of isocyanate. Suitable agents include molecular sieves, for example, zeolite 3.ANG., or magnesium sulfate, sodium sulfate, and/or calcium chloride.
A third method involves condensation of product isocyanate in the presence of a substantially water-immiscible solvent, optionally in the presence of water absorbing agents. However, such "solvent-borne" isocyanate recovery processes require an additional partial condensation step to separate isocyanate from the recovery solvent, adding to overall process complexity and cost.
The present invention provides an improved method for separating water from isocyanate and water-containing process streams resulting from gas-phase oxidative dehydrogenation of formamides. The present invention offers an efficient, economic product recovery process providing high product recovery yields and ready adaptability to continuous operation. The process of the present invention is particularly suited for use in production of alkyl isocyanates such as methyl isocyanate.